Mobile phone systems have a variety of frequency ranges from 70 MHz to 5 GHz. In order to process these multiple frequency ranges, one terminal device in a conventional system is required to have RF (Radio Frequency) circuits. Each RF circuit transmits and receives signals in a different frequency range, respectively.
For downsizing and reducing the thickness of the terminal device, an area of hardware is required to be reduced as small as possible. However, it is problem that the area of the circuits becomes extremely large because RF circuits for multiple frequency ranges must be provided in above method. Therefore it is tried to simplify the system by commonly using a variable capacity element as matching element, formed with MEMS (Micro Electro Mechanical System) technology.
Japanese Patent Application Publication (Kokai) No. 2004-32788 discloses such variable capacity element that has a pair of capacitor electrodes having square shape with parallel plates, and drive electrodes connected with one of capacitor electrodes mechanically. Electrostatic attractive force is generated by applying voltage between driver electrodes in such variable capacity element. Capacitance of the variable capacity element is formed by a pair of capacitor electrodes, and is variable with a change of a distance of electrodes, by moving one of capacitor electrodes which is connected with one of the driver electrodes mechanically. If capacitance exists in a region other than the capacitor electrodes, a ratio of variability of the variable capacity element is decreased. Therefore capacitor wirings connected with capacitor electrodes respectively, are arranged to intersect with right angles to each other to eliminate above capacitance in the region other than the capacitor electrodes.
It is required that an inductance of the capacitor should be low to prevent from decreasing its self-resonant frequency, in a case of increasing the area of the capacitor electrodes to cope with a request of increasing its capacitance. For example, to suppress the self-resonant frequency in 5 GHz, it is sufficient that the effective inductance is below 1 nH when the effective capacitance is 1 pF. Meanwhile, when the effective capacitance is increased to 10 pF, the effective inductance is required to be lowered below 100 pF. However, conventionally capacitor wirings connected with capacitor electrodes are arranged to intersect at right angles to each other, so that the inductance is large. Therefore it is difficult to attain the effective inductance is below 100 pF, considering whole variable capacity element should be molded.
Furthermore whole variable capacity element has to be sealed with a hollow sealing architecture to have mobility of the capacitor electrode. It is preferable that the sealing architecture is manufactured in a semiconductor wafer process when the variable capacity element is manufactured in the same process. Therefore it is proposed to manufacture the architecture by a thin insulated film. However, if the area of the capacitor electrode increases, the sealing architecture becomes so large. Therefore mechanical strength cannot be obtained so that it is problem that the variable capacity element breaks down.
As stated above, increasing the capacitance of the variable capacity element was difficult because reduction of inductance and holding the mechanical strength of the sealing architecture is difficult.